The invention relates to a control valve for adjusting the cross-sectional area of flow in a pipe, in particular for highly dynamic control of the cooling water volume for cooling sections in rolling mills.
A control valve is a device which operates with auxiliary power and controls flow rate in a process system.
In wire and bar stock steel rolling mills, water-cooling sections are used to influence the mechanical properties of the material. Various methods are used for this purpose:
The Tempcore method is essentially used in the production of structural steel and involves using the rolling heat to heat-treat the finished rolled product upstream of the cooling bed (bar stock steel) or the laying head (wire). The rolled stock leaves the final roll stand at a temperature of 950-1050° C. In a cooling section, the outer layer is cooled by high-pressure water (5-16 bar) to temperatures of <400° C. This results in a martensitic structure. During subsequent temperature equalization, on the one hand, the outer martensitic layer is reheated (tempered) and, on the other hand, the core is cooled thus giving rise to a ferritic/pearlitic grain structure. Using the Tempcore method makes it possible to achieve the required mechanical properties of the finished product together with a simultaneous significant reduction in the addition of alloy elements. Accurate temperature control determines the potential savings in alloy elements.
In thermomechanical rolling, the rolled stock is cooled to a defined temperature before the final shaping steps. This results in a fine grain structure which simultaneously combines high strength, good toughness and cold workability. Various forms are used: thermomechanical rolling gives rise to mechanical properties which could otherwise only be achieved by addition of alloy elements. Accurate temperature control is a prerequisite for application of thermomechanical rolling methods.
Heating of the continuous-cast billets in the reheating furnace upstream of the hot-rolling mill is never completely uniform over the length of the billet. The purpose of the reheating furnace immediately downstream of the continuous caster is to reheat the continuous-cast billets which have cooled down. Water-cooled rails on which the billets rest during heating are required to support the continuous-cast billets in the reheating furnace. Shaded areas, which are manifested as a temperature profile over the length of the billets, occur in the region of the rails. Avoiding temperature deviations over the length of the billets entails highly dynamic control of the cooling water.
Commercially available control elements for controlling the cooling water can be divided into pneumatic and electrical control elements:
Control elements with a pneumatic actuator respond in a short to moderate response time. The problem is, however, that the transition from static to sliding friction results in a stick-slip effect which results in non-reproducible response behavior and thus inconsistent response times. Pneumatic actuators are accordingly unsuitable for high-precision, dynamic control processes.
Control elements with an electrical actuator are based on a self-locking geared motor unit with short response times and high positioning precision but excessively low positioning speeds.
While what is currently the fastest valve actuator on the market with a positioning speed of at most 10 mm/s does indeed achieve the required response time, it cannot however achieve the required positioning speed (50 mm/s).
It is therefore not currently possible to ensure accurate temperature control with minimum deviations in temperature from the target temperature downstream of the cooling section. Such temperature control can, however, be achieved according to the invention by highly dynamic control of the cooling water volume in conjunction with a suitable control element for controlling the cooling water.